Shuttered doors are often used to protect cash dispensing slots in automated teller machines. Various mechanisms are used to activate such shutters. One example is a worm drive. This has a screw-threaded rod that engages with a toothed cog that is attached to the shutter door. The threaded rod is rotatable in a clockwise or anticlockwise direction, the thread of the rod engaging the teeth of the cog to transfer rotation of the rod to the cog and thereby open or close the shutter.
A problem with known worm drive arrangements is that if an intruder tried to force open the closed shutter door of a cash dispensing slot, this would rotate the cog, the teeth of the cog pushing against the sloping threads of the rod in order to turn the rod and allow the shutter doors to be opened. To counter this, some prior art shutter operating mechanisms have separate locks, which engage with the drive.